Minimising losses caused by Zucchini yellow mosaic virus in vegetable cucurbit crops in tropical, sub-tropical and Mediterranean environments through cultural methods and host resistance

Viral Threats to Fruit and Vegetable Crops in the Caribbean

Viruses pose major global challenges to crop production as infections reduce the yield and quality of harvested products, hinder germplasm exchange, increase financial inputs, and threaten food security. Small island or archipelago habitat conditions such as those in the Caribbean are particularly susceptible as the region is characterized by high rainfall and uniform, warm temperatures throughout the year. Moreover, Caribbean islands are continuously exposed to disease risks because of their location at the intersection of transcontinental trade between North and South America and their role as central hubs for regional and global agricultural commodity trade. This review provides a summary of virus disease epidemics that originated in the Caribbean and those that were introduced and spread throughout the islands. Epidemic-associated factors that impact disease development are also discussed. Understanding virus disease epidemiology, adoption of new diagnostic technologies, implementation of biosafety protocols, and widespread acceptance of biotechnology solutions to counter the effects of cultivar susceptibility remain important challenges to the region. Effective integrated disease management requires a comprehensive approach that should include upgraded phytosanitary measures and continuous surveillance with rapid and appropriate responses.

Zucchini Yellow Mosaic Virus (ZYMV) as a Serious Biotic Stress to Cucurbits: Prevalence, Diversity, and Its Implications for Crop Sustainability

Zucchini yellow mosaic virus (ZYMV) is a severe threat to cucurbit crops worldwide, including Pakistan. This study was pursued to evaluate the prevalence, geographic distribution, and molecular diversity of ZYMV isolates infecting cucurbits in Pakistan's Pothwar region. Almost all the plant viruses act as a biotic stress on the host plants, which results in a yield loss. These viruses cause losses in single-infection or in mixed-infection cucurbit crops, and we have found a number of mixed-infected samples belonging to the Curubitaceae family. Serological detection of the tested potyviruses in the collected cucurbit samples revealed that ZYMV was the most prevalent virus, with a disease incidence (DI) at 35.2%, followed by Papaya ringspot virus (PRSV) with an incidence of 2.2%, and Watermelon mosaic virus (WMV) having an incidence as little as 0.5% in 2016. In the year 2017, a relatively higher disease incidence of 39.7%, 2.4%, and 0.3% for ZYMV, WMV, and PRSV, respectively, was recorded. ZYMV was the most prevalent virus with the highest incidence in Attock, Rawalpindi, and Islamabad, while PRSV was observed to be the highest in Islamabad and Jhelum. WMV infection was observed only in Rawalpindi and Chakwal. Newly detected Pakistani ZYMV isolates shared 95.8-97.0% nucleotide identities among themselves and 77.1-97.8% with other isolates retrieved from GenBank. Phylogenetic relationships obtained using different ZYMV isolates retrieved from GenBank and validated by in silico restriction analysis revealed that four Pakistani isolates clustered with other ZYMV isolates in group IIb with Chinese, Italian, Polish, and French isolates, while another isolate (MK848239) formed a separate minor clade within IIb. The isolate MK8482490, reported to infect bitter gourd in Pakistan, shared a minor clade with a Chinese isolate (KX884570). Recombination analysis revealed that the recently found ZYMV isolate (MK848239) is most likely a recombinant of Pakistani (MK848237) and Italian (MK956829) isolates, with a recombinant breakpoint between 266 and 814 nucleotide positions. Local isolate comparison and recombination detection may aid in the development of a breeding program that identifies resistant sources against recombinant isolates because the ZYMV is prevalent in a few cucurbit species grown in the surveyed areas and causes heavy losses and economic damage to the agricultural community.

Unraveling the Mechanisms of Virus-Induced Symptom Development in Plants

Plant viruses, as obligate intracellular parasites, induce significant changes in the cellular physiology of host cells to facilitate their multiplication. These alterations often lead to the development of symptoms that interfere with normal growth and development, causing USD 60 billion worth of losses per year, worldwide, in both agricultural and horticultural crops. However, existing literature often lacks a clear and concise presentation of the key information regarding the mechanisms underlying plant virus-induced symptoms. To address this, we conducted a comprehensive review to highlight the crucial interactions between plant viruses and host factors, discussing key genes that increase viral virulence and their roles in influencing cellular processes such as dysfunction of chloroplast proteins, hormone manipulation, reactive oxidative species accumulation, and cell cycle control, which are critical for symptom development. Moreover, we explore the alterations in host metabolism and gene expression that are associated with virus-induced symptoms. In addition, the influence of environmental factors on virus-induced symptom development is discussed. By integrating these various aspects, this review provides valuable insights into the complex mechanisms underlying virus-induced symptoms in plants, and emphasizes the urgency of addressing viral diseases to ensure sustainable agriculture and food production.

Plant Viruses of Agricultural Importance: Current and Future Perspectives of Virus Disease Management Strategies

Plant viruses cause significant losses in agricultural crops worldwide, affecting the yield and quality of agricultural products. The emergence of novel viruses or variants through genetic evolution and spillover from reservoir host species, changes in agricultural practices, mixed infections with disease synergism, and impacts from global warming pose continuous challenges for the management of epidemics resulting from emerging plant virus diseases. This review describes some of the most devastating virus diseases plus select virus diseases with regional importance in agriculturally important crops that have caused significant yield losses. The lack of curative measures for plant virus infections prompts the use of risk-reducing measures for managing plant virus diseases. These measures include exclusion, avoidance, and eradication techniques, along with vector management practices. The use of sensitive, high throughput, and user-friendly diagnostic methods is crucial for defining preventive and management strategies against plant viruses. The advent of next-generation sequencing technologies has great potential for detecting unknown viruses in quarantine samples. The deployment of genetic resistance in crop plants is an effective and desirable method of managing virus diseases. Several dominant and recessive resistance genes have been used to manage virus diseases in crops. Recently, RNA-based technologies such as dsRNA- and siRNA-based RNA interference, microRNA, and CRISPR/Cas9 provide transgenic and nontransgenic approaches for developing virus-resistant crop plants. Importantly, the topical application of dsRNA, hairpin RNA, and artificial microRNA and trans-active siRNA molecules on plants has the potential to develop GMO-free virus disease management methods. However, the long-term efficacy and acceptance of these new technologies, especially transgenic methods, remain to be established.

Protective and Curative Activities of Paenibacillus polymyxa against Zucchini yellow mosaic virus Infestation in Squash Plants

The use of microbial products as natural biocontrol agents to increase a plant's systemic resistance to viral infections is a promising way to make agriculture more sustainable and less harmful to the environment. The rhizobacterium Paenibacillus polymyxa has been shown to have strong biocontrol action against plant diseases, but its antiviral activity has been little investigated. Here, the efficiency of the culture filtrate of the P. polymyxa strain SZYM (Acc# ON149452) to protect squash (Cucurbita pepo L.) plants against a Zucchini yellow mosaic virus (ZYMV, Acc# ON159933) infection was evaluated. Under greenhouse conditions, the foliar application of the culture filtrate of SZYM either in protective or curative treatment conditions enhanced squash growth, reduced disease severity, and decreased ZYMV accumulation levels in the treated plants when compared to the non-treated plants. The protective treatment group exhibited the highest inhibitory effect (80%), with significant increases in their total soluble carbohydrates, total soluble protein content, ascorbic acid content, and free radical scavenging activity. Furthermore, a considerable increase in the activities of reactive oxygen species scavenging enzymes (superoxide dismutase, polyphenol oxidase, and peroxidase) were also found. In addition, the induction of systemic resistance with a significant elevation in the transcriptional levels of polyphenolic pathway genes (CHS, PAL, and C3H) and pathogenesis-related genes (PR-1 and PR-3) was observed. Out of the 14 detected compounds in the GC-MS analysis, propanoic acid, benzenedicarboxylic acid, tetradecanoic acid, and their derivatives, as well as pyrrolo [1,2-a] pyrazine-1,4-dione, hexahydro-3-(2-methylpropyl) were the primary ingredient compounds in the ethyl acetate extract of the SZYM-culture filtrate. Such compounds may act as elicitor molecules that induce systemic resistance against viral infection. Consequently, P. polymyxa can be considered a powerful plant growth-promoting bacterium (PGPB) in agricultural applications as well as a source of bioactive compounds for sustainable disease management. As far as we know, this is the first time that P. polymyxa has been shown to fight viruses in plants.

Effect of Breathable Row Covers and Ground Cover on Pest Insect Levels and Cucurbit Yield

Organic control measures in muskmelon and squash production are part of an integrated pest management approach that can include using floating row covers, generalist predators, and ground cover. These are used in Kentucky, allowing for a reduction in insecticide use and diminished virus incidence while increasing yield. Commonly used row covers are made from spunbonded fabric that retains heat and must be removed at anthesis and kept off until the end of the season. Thus, a new farming regime containing breathable mesh covers which can be replaced after anthesis was tested for longer season insect exclusion across two growing seasons. Additionally, ground cover treatments, consisting of mulch or bare ground were tested for their effect on pest insect abundance and fruit yield. Pest insect numbers were usually lower in plots with mesh row covers and in some cases, mulch ground cover also contributed to lower pest numbers. A stronger impact on pest numbers was observed in melon than squash. Melon yield was always significantly higher in plots with mesh row covers and mulch ground cover. This trend was not observed with squash in 2014 but was true in 2015. In 2015, most plants under the fabric row covers died because of high temperatures immediately after transplanting highlighting the need for breathable mesh row covers.

Detection of plant virus particles with a capacitive field-effect sensor

Plant viruses are major contributors to crop losses and induce high economic costs worldwide. For reliable, on-site and early detection of plant viral diseases, portable biosensors are of great interest. In this study, a field-effect SiO2-gate electrolyte-insulator-semiconductor (EIS) sensor was utilized for the label-free electrostatic detection of tobacco mosaic virus (TMV) particles as a model plant pathogen. The capacitive EIS sensor has been characterized regarding its TMV sensitivity by means of constant-capacitance method. The EIS sensor was able to detect biotinylated TMV particles from a solution with a TMV concentration as low as 0.025 nM. A good correlation between the registered EIS sensor signal and the density of adsorbed TMV particles assessed from scanning electron microscopy images of the SiO2-gate chip surface was observed. Additionally, the isoelectric point of the biotinylated TMV particles was determined via zeta potential measurements and the influence of ionic strength of the measurement solution on the TMV-modified EIS sensor signal has been studied.

Host plant affiliations of aphid vector species found in a remote tropical environment

The Ord River Irrigation Area (ORIA) produces annual crops during the dry season (April to October), and perennial crops all-year-round, and is located in tropical northwestern Australia. Sandalwood plantations cover 50 % of the ORIA's cropping area. Aphids cause major crop losses through transmission of viruses causing debilitating diseases and direct feeding damage. During 2016-2017, in both dry and wet seasons a total of 3320 leaf samples were collected from diverse types of sites on cultivated and uncultivated land and 1248 (38 %) of them were from aphid-colonized plants. In addition, aphids were found at 236 of 355 sampling sites. The 62 plant species sampled came from 23 families 19 of which contained aphid-colonized species. Aphid hosts included introduced weeds, Australian native plants, and volunteer or planted crop plants. Six aphid species were identified by light microscopy and CO1 gene sequencing, but there was no within species nucleotide sequence diversity. Aphis nerii, Hysteroneura setariae, Rhopalosiphum maidis and Schoutedenia ralumensis each colonized 1-3 plant species from a single plant family. A. craccivora colonized 14 species in five plant families. A. gossypii was the most polyphagous species colonizing 19 species in 11 plant families. A. gossypii, A. craccivora, A. nerii and S. ralumensis were found in both wet and dry seasons. Because of A. craccivora's prevalence and high incidences on understory weeds and host trees, sandalwood plantations were important reservoirs for aphid spread to wild and crop plant hosts growing in cultivated and uncultivated land. Alternative hosts growing in rural bushland, irrigation channel banks, vacant or fallow land, and orchard plantation understories also constituted significant aphid reservoirs. This study provides new knowledge of the ecology of aphid vector species not only in the ORIA but also in tropical northern Australia generally. It represents one of relatively few investigations on aphid ecology in tropical environments worldwide.

Epidemiology of Zucchini yellow mosaic virus in cucurbit crops in a remote tropical environment

In the remote Ord River Irrigation Area (ORIA) in tropical northwest Australia, severe Zucchini yellow mosaic virus (ZYMV) epidemics threaten dry season (April-October) cucurbit crops. In 2016-2017, wet season (November-March) sampling studies found a low incidence ZYMV infection in wild Cucumis melo and Citrullus lanatus var. citroides plants, and both volunteer and garden crop cucurbits. Such infections enable its persistence in the wet season, and act as reservoirs for its spread to commercial cucurbit crops during the dry season. Tests on 1019 samples belonging to 55 species from 23 non-cucurbitaceous plant families failed to detect ZYMV. It was also absent from wild cucurbit weeds within sandalwood plantations. The transmission efficiencies of a local isolate by five aphid species found in the ORIA were: 10 % (Aphis craccivora), 7% (A. gossypii), 4% (A. nerii), and 0% (Rhopalosiphum maidis and Hysteroneura setariae). In 2016-2017, in all-year-round trapping at five representative sites, numbers of winged aphids caught were greatest in July-August (i.e. mid growing season) but varied widely between trap sites reflecting local aphid host abundance and year. Apart from one localised exception in 2017, flying aphid numbers caught and ZYMV spread in data collection blocks during 2015-2017 resembled what occurred commercial cucurbit crops. When ZYMV spread from external infection sources into melon blocks, its predominant spread pattern consisted of 1 or 2 plant infection foci often occurring at their margins. In addition, when plants of 29 cucurbit cultivars were inoculated with an ORIA isolate and two other ZYMV isolates and the phenotypes elicited were compared, they resembled each other in overall virulence. However, depending upon isolate-cultivar combination, differences in symptom expression and severity occurred, and one isolate caused a systemic hypersensitive phenotype in honeydew melon cvs Estilo and Whitehaven. When the new genomic RNA sequences of 19 Australian isolates were analysed, all seven ORIA isolates fitted within ZYMV phylogroup B, which also included two from southwest Australia, whereas the remaining 10 isolates were all within minor phylogroups A-I or A-II. Based on previous research and the additional knowledge of ZYMV epidemic drivers established here, an integrated disease management strategy targeting ZYMV spread was devised for the ORIA's cucurbit industry.

Global Dimensions of Plant Virus Diseases: Current Status and Future Perspectives

Viral diseases provide a major challenge to twenty-first century agriculture worldwide. Climate change and human population pressures are driving rapid alterations in agricultural practices and cropping systems that favor destructive viral disease outbreaks. Such outbreaks are strikingly apparent in subsistence agriculture in food-insecure regions. Agricultural globalization and international trade are spreading viruses and their vectors to new geographical regions with unexpected consequences for food production and natural ecosystems. Due to the varying epidemiological characteristics of diverent viral pathosystems, there is no one-size-fits-all approach toward mitigating negative viral disease impacts on diverse agroecological production systems. Advances in scientific understanding of virus pathosystems, rapid technological innovation, innovative communication strategies, and global scientific networks provide opportunities to build epidemiologic intelligence of virus threats to crop production and global food security. A paradigm shift toward deploying integrated, smart, and eco-friendly strategies is required to advance virus disease management in diverse agricultural cropping systems.

Zucchini yellow mosaic virus Genomic Sequences from Papua New Guinea: Lack of Genetic Connectivity with Northern Australian or East Timorese Genomes, and New Recombination Findings

Zucchini yellow mosaic virus (ZYMV) isolates were obtained in Papua New Guinea (PNG) from cucumber (Cucumis sativus) or pumpkin (Cucurbita spp.) plants showing mosaic symptoms growing at Kongop in the Mount Hagen District, Western Highlands Province, or Zage in the Goroka District, Eastern Highlands Province. The samples were blotted onto FTA cards, which were sent to Australia, where they were subjected to high-throughput sequencing. When the coding regions of the nine new ZYMV genomic sequences found were compared with those of 64 other ZYMV sequences from elsewhere, they grouped together, forming new minor phylogroup VII within ZYMV's major phylogroup A. Genetic connectivity was lacking between ZYMV genomic sequences from PNG and its neighboring countries, Australia and East Timor; the closest match between a PNG and any other genomic sequence was a 92.8% nucleotide identity with a sequence in major phylogroup A's minor phylogroup VI from Japan. When the RDP5.2 recombination analysis program was used to compare 66 ZYMV sequences, evidence was obtained of 30 firm recombination events involving 41 sequences, and all isolates from PNG were recombinants. There were 21 sequences without recombination events in major phylogroup A, whereas there were only 4 such sequences within major phylogroup B. ZYMV's P1, Cl, N1a-Pro, P3, CP, and NIb regions contained the highest evidence of recombination breakpoints. Following removal of recombinant sequences, seven minor phylogroups were absent (I, III, IV, V, VI, VII, and VIII), leaving only minor phylogroups II and IX. By contrast, when a phylogenetic tree was constructed using recombinant sequences with their recombinationally derived tracts removed before analysis, five previous minor phylogroups remained unchanged within major phylogroup A (II, III, IV, V, and VII) while four formed two new merged phylogroups (I/VI and VIII/IX). Absence of genetic connectivity between PNG, Australian, and East Timorese ZYMV sequences, and the 92.8% nucleotide identity between a PNG sequence and the closest sequence from elsewhere, suggest that a single introduction may have occurred followed by subsequent evolution to adapt to the PNG environment. The need for enhanced biosecurity measures to protect against potentially damaging virus movements crossing the seas separating neighboring countries in this region of the world is discussed.

Genetic Connectivity Between Papaya Ringspot Virus Genomes from Papua New Guinea and Northern Australia, and New Recombination Insights

Isolates of papaya ringspot virus (PRSV) were obtained from plants of pumpkin (Cucurbita spp.) or cucumber (Cucumis sativus) showing mosaic symptoms growing at Zage in Goroka District in the Eastern Highland Province of Papua New Guinea (PNG) or Bagl in the Mount Hagen District, Western Highlands Province. The samples were sent to Australia on FTA cards where they were subjected to High Throughput Sequencing (HTS). When the coding regions of the six new PRSV genomic sequences obtained via HTS were compared with those of 54 other complete PRSV sequences from other parts of the world, all six grouped together with the 12 northern Australian sequences within major phylogroup B minor phylogroup I, the Australian sequences coming from three widely dispersed locations spanning the north of the continent. Notably, none of the PNG isolates grouped with genomic sequences from the nearby country of East Timor in phylogroup A. The closest genetic match between Australian and PNG sequences was a nucleotide (nt) sequence identity of 96.9%, whereas between PNG and East Timorese isolates it was only 83.1%. These phylogenetic and nt identity findings demonstrate genetic connectivity between PRSV populations from PNG and Australia. Recombination analysis of the 60 PRSV sequences available revealed evidence of 26 recombination events within 18 isolates, only four of which were within major phylogroup B and none of which were from PNG or Australia. Within the recombinant genomes, the P1, Cl, NIa-Pro, NIb, 6K2, and 5'UTR regions contained the highest numbers of recombination breakpoints. After removal of nonrecombinant sequences, four minor phylogroups were lost (IV, VII, VIII, XV), only one of which was in phylogroup B. When genome regions from which recombinationally derived tracts of sequence were removed from recombinants prior to alignment with nonrecombinant genomes, seven previous minor phylogroups within major phylogroup A, and two within major phylogroup B, merged either partially or entirely forming four merged minor phylogroups. The genetic connectivity between PNG and northern Australian isolates and absence of detectable recombination within either group suggests that PRSV isolates from East Timor, rather than PNG, might pose a biosecurity threat to northern Australian agriculture should they prove more virulent than those already present.

Zucchini yellow mosaic virus Populations from East Timorese and Northern Australian Cucurbit Crops: Molecular Properties, Genetic Connectivity, and Biosecurity Implications

Zucchini yellow mosaic virus (ZYMV) isolates from cucurbit crops growing in northern Australia and East Timor were investigated to establish possible genetic connectivity between crop viruses in Australia and Southeast Asia. Leaves from symptomatic plants of pumpkin (Cucurbita moschata and C. maxima), melon (Cucumis melo), and zucchini (C. pepo) were sampled near Broome, Darwin, and Kununurra in northern Australia. Leaves from symptomatic plants of cucumber (C. sativus) and pumpkin sampled in East Timor were sent to Australia on FTA cards. These samples were subjected to high-throughput sequencing and 15 complete new ZYMV genomic sequences obtained. When their nucleotide sequences were compared with those of 48 others from GenBank, the East Timorese and Kununurra sequences (three per location) and single earlier sequences from Singapore and Reunion Island were all in major phylogroup B. The seven Broome and two Darwin sequences were in minor phylogroups I and II, respectively, within larger major phylogroup A. When coat protein (CP) nucleotide sequences from the 15 new genomes and 47 Australian isolates sequenced previously were compared with 331 other CP sequences, the closest genetic match for a sequence from Kununurra was with an East Timorese sequence (95.5% nucleotide identity). Analysis of the 63 complete genomes found firm recombination events in 12 (75%) and 2 (4%) sequences from northern Australia or Southeast Asia versus the rest of the world, respectively; therefore, the formers' high recombination frequency might reflect adaptation to tropical conditions. Both parents of the recombinant Kununurra sequence were East Timorese. Phylogenetic analysis, nucleotide sequence identities, and recombination analysis provided clear evidence of genetic connectivity between sequences from Kununurra and East Timor. Inoculation of a Broome isolate to zucchini and watermelon plants reproduced field symptoms observed in northern Australia. This research has important biosecurity implications over entry of damaging viral crop pathogens not only into northern Australia but also moving between Australia's different agricultural regions.

Papaya ringspot virus Populations From East Timorese and Northern Australian Cucurbit Crops: Biological and Molecular Properties, and Absence of Genetic Connectivity

To examine possible genetic connectivity between crop viruses found in Southeast Asia and Australia, Papaya ringspot virus biotype W (PRSV-W) isolates from cucurbits growing in East Timor and northern Australia were studied. East Timorese samples from cucumber (Cucumis sativus) or pumpkin (Cucurbita moschata and C. maxima) were sent to Australia on FTA cards. These samples and others of pumpkin, rockmelon, honeydew melon (Cucumis melo), or watermelon (Citrullus lanatus) growing in one location each in northwest, north, or northeast Australia were subjected to high throughput sequencing (HTS). When the 17 complete PRSV genomic sequences obtained by HTS were compared with 32 others from GenBank, the five from East Timor were in a different major phylogroup from the 12 Australian sequences. Moreover, the East Timorese and Australian sequences each formed their own minor phylogroups named VI and I, respectively. A Taiwanese sequence was closest to the East Timorese (89.6% nt dentity), and Mexican and Brazilian sequences were the closest to the Australian (92.3% nt identity). When coat protein gene (CP) sequences from the 17 new genomic sequences were compared with 126 others from GenBank, three Australian isolates sequenced more than 20 years ago grouped with the new Australian sequences, while the closest sequence to the East Timorese was from Thailand (93.1% nt identity). Recombination analysis revealed 13 recombination events among the 49 complete genomes. Two isolates from East Timor (TM50, TM32) and eight from GenBank were recombinants, but all 12 Australian isolates were non-recombinants. No evidence of genome connectivity between Australian and Southeast Asian PRSV populations was obtained. The strand-specific RNA library approach used optimized data collection for virus genome assembly. When an Australian PRSV isolate was inoculated to plants of zucchini (Cucurbita pepo), watermelon, rockmelon, and honeydew melon, they all developed systemic foliage symptoms characteristic of PRSV-W, but symptom severity varied among melon cultivars.

Control of cucurbit viruses

More than 70 well-characterized virus species transmitted by a diversity of vectors may infect cucurbit crops worldwide. Twenty of those cause severe epidemics in major production areas, occasionally leading to complete crop failures. Cucurbit viruses' control is based on three major axes: (i) planting healthy seeds or seedlings in a clean environment, (ii) interfering with vectors activity, and (iii) using resistant cultivars. Seed disinfection and seed or seedling quality controls guarantee growers on the sanitary status of their planting material. Removal of virus or vector sources in the crop environment can significantly delay the onset of viral epidemics. Insecticide or oil application may reduce virus spread in some situations. Diverse cultural practices interfere with or prevent vector reaching the crop. Resistance can be obtained by grafting for soil-borne viruses, by cross-protection, or generally by conventional breeding or genetic engineering. The diversity of the actions that may be taken to limit virus spread in cucurbit crops and their limits will be discussed. The ultimate goal is to provide farmers with technical packages that combine these methods within an integrated disease management program and are adapted to different countries and cropping systems.

Zucchini yellow mosaic virus: Contact Transmission, Stability on Surfaces, and Inactivation with Disinfectants

In glasshouse experiments, Zucchini yellow mosaic virus (ZYMV) was transmitted from infected to healthy zucchini (Cucurbita pepo) plants by direct contact when leaves were rubbed against each other, crushed, or trampled, and, to a lesser extent, on ZYMV-contaminated blades. When sap from zucchini plants infected with three ZYMV isolates was kept at room temperature for up to 6 h, it infected healthy plants readily. Also, when sap from ZYMV-infected leaves was applied to seven surfaces (cotton, plastic, leather, metal, rubber vehicle tire, rubber-soled footwear, and human skin) and left for up to 48 h before the ZYMV-contaminated surface was rubbed onto healthy zucchini plants, ZYMV remained infective for 48 h on tire, 24 h on plastic and leather, and up to 6 h on cotton, metal, and footwear. On human skin, ZYMV remained infective for 5 min only. The effectiveness of 13 disinfectants at inactivating ZYMV was evaluated by adding them to sap from ZYMV-infected leaves which was then rubbed on to healthy zucchini plants. None of the plants became infected when nonfat dried milk (20%, wt/vol) or bleach (sodium hypochlorite at 42 g/liter, diluted 1:4) were used. When ZYMV-infected pumpkin leaves were trampled by footwear and then used to trample healthy plants, all plants became infected; however, when contaminated footwear was dipped in a footbath containing bleach (sodium hypochlorite at 42 g/liter, diluted 1:4) before trampling, none became infected. This study demonstrates that ZYMV can be transmitted by contact and highlights the need for on-farm hygiene practices (decontaminating tools, machinery, clothing, and so on) to be included in integrated disease management strategies for ZYMV in cucurbit crops.

Indigenous and introduced potyviruses of legumes and Passiflora spp. from Australia: biological properties and comparison of coat protein nucleotide sequences

Five Australian potyviruses, passion fruit woodiness virus (PWV), passiflora mosaic virus (PaMV), passiflora virus Y, clitoria chlorosis virus (ClCV) and hardenbergia mosaic virus (HarMV), and two introduced potyviruses, bean common mosaic virus (BCMV) and cowpea aphid-borne mosaic virus (CAbMV), were detected in nine wild or cultivated Passiflora and legume species growing in tropical, subtropical or Mediterranean climatic regions of Western Australia. When ClCV (1), PaMV (1), PaVY (8) and PWV (5) isolates were inoculated to 15 plant species, PWV and two PaVY P. foetida isolates infected P. edulis and P. caerulea readily but legumes only occasionally. Another PaVY P. foetida isolate resembled five PaVY legume isolates in infecting legumes readily but not infecting P. edulis. PaMV resembled PaVY legume isolates in legumes but also infected P. edulis. ClCV did not infect P. edulis or P. caerulea and behaved differently from PaVY legume isolates and PaMV when inoculated to two legume species. When complete coat protein (CP) nucleotide (nt) sequences of 33 new isolates were compared with 41 others, PWV (8), HarMV (4), PaMV (1) and ClCV (1) were within a large group of Australian isolates, while PaVY (14), CAbMV (1) and BCMV (3) isolates were in three other groups. Variation among PWV and PaVY isolates was sufficient for division into four clades each (I-IV). A variable block of 56 amino acid residues at the N-terminal region of the CPs of PaMV and ClCV distinguished them from PWV. Comparison of PWV, PaMV and ClCV CP sequences showed that nt identities were both above and below the 76-77% potyvirus species threshold level. This research gives insights into invasion of new hosts by potyviruses at the natural vegetation and cultivated area interface, and illustrates the potential of indigenous viruses to emerge to infect introduced plants.